1. Field of the Invention
The present invention relates to an MR (MagnetoResistive) device with a ferromagnetic tunnel junction implemented by an insulation layer and ferromagnetic layers sandwiching the insulation layer, and a method of producing the same.
2. Description of the Background Art
A ferromagnetic tunnel junction has a tunnel barrier layer and two ferromagnetic layers sandwiching the tunnel barrier layer. The tunnel barrier layer is implemented as a several nanometers thick insulator. Electric resistance between the two ferromagnetic layers varies in accordance with a relative angle between the magnetizations of two ferromagnetic materials. More specifically, the electric resistance is minimum when the magnetizations are parallel or maximum when they are not parallel.
The difference of the electric resistance between the parallel state and the non-parallel state of the two magnetizations is generally represented by an MR ratio. Specifically, assume that the two ferromagnetic materials have spin polarization P1 and P2, respectively. Then, the MR ratio is expressed as 2(P1·P2)/(1−P1·P2). This shows that the above difference increases with an increase in the spin polarization of the two ferromagnetic materials. By using the dependence of the electric resistance on the relative angle, it is possible to produce an MR device for sensing the variation of an external magnetic field in terms of the variation of the electric resistance.
Technologies for allowing a ferromagnetic tunnel junction to be applied to an MR device include one that has been customary with a spin valve. A spin valve includes two ferromagnetic layers magnetically isolated from each other by a nonmagnetic layer. An antiferromagnetic layer is stacked on one of the ferromagnetic layers. The ferromagnetic layer underlying the antiferromagnetic layer is a fixed layer whose direction of magnetization is fixed by an interchange-couple magnetic field. The other ferromagnetic layer is a free layer whose direction of magnetization varies in accordance with the external magnetic field. In this configuration, the direction of magnetization of the free layer spins due to the external magnetic field, but the magnetization of the fixed layer does not spin. This makes it possible to vary the relative angle between the magnetization of the free layer and that of the fixed layer on the basis of the external magnetic field. Assuming that the nonmagnetic layer intervening between the ferromagnetic layers is the tunnel barrier layer, then the spin valve technology is applicable to the ferromagnetic tunnel junction. The variation of the external magnetic field can be sensed in terms of the variation of tunnel resistance.
To form the tunnel barrier layer of a ferromagnetic tunnel junction, it is a common practice to form a metal or semiconductor film, which is several nanometers thick or less, on a ferromagnetic layer and then oxidize the metal or the semiconductor for thereby making it insulative. In many cases, use is made of aluminum desirably adhering to an underlying magnetic layer and capable of forming a tunnel barrier layer having a desirable covering ability.
Methods available for oxidizing aluminum are generally classified into a native oxidizing method using oxygen of a ground level and a method using a radical, plasma or similar oxygen of an excitation level, as will be described hereinafter.
The native oxidizing method forms an aluminum film on an underlying magnetic material and then holds them in an oxygen gas atmosphere to thereby oxidize the aluminum film. The resulting tunnel barrier layer, which has a low barrier and a low junction resistance, is applied to a ferromagnetic tunnel junction for a magnetic head. H. Tsuge et al., for (example, report a ferromagnetic tunnel junction using such a tunnel barrier layer in “Materials Research Society Symposium Proceedings”, Vol. 517, 87 (1998). The junction taught in this report has a standardized junction resistance of 240 Ω.μm2 and an MR ratio of 12%.
The method using oxygen of the excitation level implements a tunnel barrier layer whose junction resistance is as high as the order of kΩ.μm2 or above. S. Parkin et al., for example, teaches a tunnel barrier layer with a ferromagnetic tunnel junction having a junction resistance of 11 MΩ.μm2 in “Journal of Applied Physics”, 85, 5828 (1999). To form the tunnel barrier layer, aluminum is oxidized by oxygen plasma.
Another possible method of forming a tunnel barrier layer is directly depositing aluminum oxide. This method, however, has a problem that the barrier characteristics deteriorate due to the loss of oxygen. Another problem is that aluminum oxide is less adhesive to the underlying layer than aluminum, resulting in micro-defects.
Ideally, the MR ratio of a ferromagnetic tunnel junction is dependent on the spin polarization of a ferromagnetic material, as stated above. In practice, however, the interface between the ferromagnetic material and the tunnel barrier layer or insulation layer is not perfect. For example, in a tunnel barrier layer using aluminum oxide, it is likely that non-oxidized aluminum remains due to short oxygen or even the underlying ferromagnetic layer is oxidized due to excessive oxidation. As a result, the effective spin polarization of the ferromagnetic material and therefore the MR ratio decreases.
The native oxidizing method causes oxygen of the ground level to slowly oxidize, e.g., a metal and therefore oxidizes a ferromagnetic material underlying the metal little. However, an oxidizing force available with this method is weak and apt to leave a non-oxidized aluminum layer between an aluminum oxide layer and a ferromagnetic layer underlying it. As a result, spin diffusion occurs in electrons migrating between the ferromagnetic layer and a ferromagnetic layer, which overlies the aluminum oxide layer, on the basis of the tunnel effect. The spin diffusion lowers the MR ratio.
By contrast, the method using plasma or similar oxygen of the excitation level exerts an oxidizing force strong enough to leave a minimum of non-oxidized aluminum. This method therefore provides a ferromagnetic tunnel junction with a high junction resistance. However, oxidation is apt to extend to a ferromagnetic layer or bottom layer because the intense oxidizing force is difficult to control. The oxidation extended to the ferromagnetic layer forms a ferromagnetic oxide layer between the layer and an aluminum oxide layer, lowering the MR ratio. Moreover, such oxidation makes the interface between the tunnel barrier layer and the ferromagnetic layer unstable while lowering the resistivity of the ferromagnetic tunnel junction to voltage.
To realize a high quality ferromagnetic tunnel junction with a high MR ratio, it is necessary to obviate the non-oxidized aluminum, the oxidation of the underlying ferromagnetic layer, and the defects of the interface and tunnel barrier discussed above.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 11-181564, 2000-36628, 2000-91666, 2000-150984, 2000-251229, 2000-322714, 2000-331472, 2000-357829 and 2001-57450.